The invention relates generally to direct current (DC) control circuits for traveling cranes and the like.
While the invention described below is directed to a particular problem relating to remote controlled bridge (over-head-traveling) cranes, the principle of the invention is applicable in a directly analogous fashion to other types of moving equipment which is powered through contacts sliding on a bus bar or equivalent means of electrification.
Bridge cranes are useful in moving articles within a large rectangular area such as warehouse floor or assembly area in a factory. This type of crane has a moving bridge which spans a pair of stationary overhead parallel rails called the runway. The bridge has wheels (bridge trolley) which ride on the parallel rails, and the means for driving the wheels is normally carried on the bridge itself. The hoist mechanism is mounted on a hoist trolley which rides along the length of the bridge. The direction of the hoist trolley movement is perpendicular to the direction of the bridge movement. The third degree of motion is provided by raising and lowering the hoist hook by operating the hoist motor or winch. Each of the DC motors carried on the bridge to drive the bridge trolley, hoist trolley and hoist must be capable of imparting forward and reverse (up and down) motion at two speeds.
The various means of controlling the motors on the bridge in the past required a plurality of bus bars (power line) along the length of the runway for each motor. The bus bars for two motors must also be supplied along the length of the bridge itself to accommodate the movement of the hoist trolley.
In the past, three different concepts have been used for bridge crane control: (1) pendant; (2) operator cab or cage; and (3) remote station. For pendant control a manual push-button controller is suspended from the hoist trolley. For more frequent crane operation, operator cab control is used; the cab is mounted directly to the bridge so that an operator can control all of the functions from the bridge. The remote control system uses a stationary control panel from which all of the bridge crane motions are commanded from a single location on the floor.
Typically, a bridge crane with reversible two-speed hoist, hoist trolley and bridge trolley motors can be remotely controlled by wall-mounted push buttons with ten conductors of runway control electrification and seven conductors of bridge control electrification. For each motor one bus bar is required for forward motion, one for reverse motion and one for each second or higher speed of operation, plus one bus bar for common or ground to serve all motors. Total electrification for the runway thus requires the following bus bar designation: common ground, bridge forward, bridge reverse, bridge high (a higher speed), hoist trolley forward, hoist trolley reverse, hoist trolley high, hoist up, hoist down, and hoist high -- a total of ten different bus bars running the entire length of the runway. The electrification on the bridge can be designated: common ground, hoist trolley forward, hoist trolley reverse, hoist trolley high, hoist up, hoist down and hoist high -- a total of seven separate bus bars.
Considering a typical bridge span of 40 feet and runway of 100 feet, a total of 1,280 linear feet of control electrification is necessary. Including hangers, collectors (shoes) and labor, the present installation cost is on the order of $6 per linear foot. Thus a reduction in the number of runway control bus bars needed for electrification of the bridge systems would be highly desirable.